1. Field of the Invention
This invention relates to a liquid crystal optical element and to a liquid crystal device incorporating same, and is more particularly concerned with a liquid crystal optical element comprising a liquid crystal layer and at least one alignment layer for alignment and tilt angle control of adjacent liquid crystal molecules in the liquid crystal layer. This invention is particularly, but not exclusively, concerned with active liquid crystal devices, i.e. liquid crystal devices having a liquid crystal layer which is field-switchable to change an optical property of the layer.
2. Description of the Related Art
It is very well known to provide a rubbed alignment layer to control the alignment and pretilt angle of adjacent liquid crystal molecules in a liquid crystal layer. However, the control of the pretilt angle requires the use of specific combinations of the liquid crystal layer and the alignment layer. It can be difficult to achieve very high pretilt angles with sufficient control by this method.
Various other methods, which do not require rubbing of the alignment layer in contact with the liquid crystal layer, have also been previously proposed.
EP-A-0689084 discloses liquid crystal devices having multiple alignment layers. One alignment layer is a photo-orientated polymer network (PPN) layer which is cured using linearly polarised light to effect anisotropic polymerisation of a monomer. Another alignment layer is formed of a photo-cured mixture of diacrylate liquid crystal monomers. The use of such a mixture is to provide a supercoolable nematic mixture which allows the LC layer to be produced at room temperature. Such alignment layer is produced by dissolving the diacrylate liquid crystal monomers with a photoinitiator in a solvent and applying the mixture to a previously-produced rubbed lower alignment layer by spin coating. This is followed by photo-curing so as to fix the orientation imparted to the liquid crystal monomer molecules by the underlying lower alignment layer. However, there is no teaching concerning control of the tilt angle produced by any of the alignment layers.
EP-A-0 506 175 provides an alignment layer consisting of an oriented polymer network in which a non-reactive liquid crystalline material is dispersed. The alignment layer is formed from a mixture of a reactive liquid crystalline material and a non-reactive liquid crystalline material. There is no teaching concerning control of the tilt angle produced by the alignment layer.
U.S. Pat. No. 5,528,401 relates to controlling the tilt angle produced by an alignment layer. The material from which the alignment layer is formed comprises a liquid crystal material and a polymerising material. The tilt angle is controlled by polymerising the polymerisable material while applying an electric or magnetic field to control the tilt angle of the liquid crystal molecules.
In M. Schadt et al, Nature, Vol. 381 May 16, 1996, pages 212 to 215, there is disclosed a modification of the PPN technique wherein, to enable pretilt angle control, novel photo-prepolymer molecules containing a coumarin moiety are employed. These photo-prepolymer molecules not only polymerise at a polymerisable moiety at one end of the molecule, but they also cross-link by fusing together in various configurations at the pyrone ring of the coumarin moiety. However, this technique requires the use a specialised monomer and also requires directional control over the incident cross-linking UV radiation.
EP-A-0467456 discloses that the pretilt angle of liquid crystal molecules in an adjacent liquid crystal layer can be controlled by controlling the thickness of a liquid crystalline auxiliary alignment layer which is spin formed on a glass substrate to which a polyimide alignment layer has been applied. The auxiliary alignment layer contains a diacrylate liquid crystal monomer, a photoinitiator and a liquid crystal material having a surface-active group such as an aliphatic chain which is directed towards an exposed surface of the auxiliary alignment layer. This can enable relatively high pretilt angles (i.e. relatively high angles relative to the plane of the alignment layer) to be achieved for liquid crystal molecules in the adjacent active liquid crystal layer. However, control of the pretilt angle requires a precise control over the thickness of the spun layer, which is difficult.
In some instances, it is desired for the molecules of the liquid crystal active layer to align on a surface with their long molecular axes normal to the surface (so called "homeotropic alignment"). Methods of achieving homeotropic alignment of common liquid crystal materials are described by L. M. Blinov et al in "Electrooptic Effects in Liquid Crystal Materials", Springer Verlag, New York Inc--Chapter 3, 1994.
In some instances, it is desirable to achieve a slight tilt (typically 1-10.sup.0) away from the homeotropic (90.degree.) alignment. Dyed guest-host displays utilising tilted-off homeotropic alignment and negative dielectric anisotropy liquid crystal mixtures have a number of attractive features including slightly improved multiplexibility and brightness. Although this "tilted-off" homeotropic alignment can be achieved via the oblique evaporation of a layer of silicon oxide onto a glass substrate [see T. Uchida et al, Japanese journal of Applied Physics, Vol. 19, No. 11, November 1980, pp 2127-2136, T. Uchida et al, IEEE Transactions on Electronic Devices, Vol. Ed-26, No. 9, September 1979, pp 1373-1374, T. J. Scheffer, Phil. Trans. R. Soc. Lond., A 309, 189-201 (1983), and U.S. Pat. No. 4,635,051], it requires special equipment and there is generally no reliable way of reproducibly achieving such tilted-off homeotropic alignment over large areas.